1. Technical Field
The present invention relates to cooling fans, and more particularly to an impeller which helps to decrease noise generated by a cooling fan incorporating the impeller.
2. Description of Related Art
It is well known that heat is produced by electronic components such as central processing units (CPUs) during their normal operations. If the heat is not timely removed, these electronic components may overheat. Therefore, heat sinks and cooling fans are often used to cool these electronic components.
Conventionally, a cooling fan includes a housing, a stator received in the housing and an impeller being rotatable with respect to the stator. The impeller includes a hub and a plurality of blades radially and outwardly extending from the hub. When the cooling fan operates, the blades of the impeller rotate around the stator to drive an airflow to flow towards an electronic component, thus cooling the electronic component continuously. Increasing revolving speed of the impeller relatively increases the amount of the airflow, therefore a heat dissipation efficiency of the cooling fan is relatively improved. However, increasing the revolving speed may correspondingly cause a rise of a noise level generated by the cooling fan, thus making a user near the fan feel uncomfortable.
There are two main forms of noise generated by the cooling fan. One is rotary noise, which is generated when the blades cut air. The rotary noise is based on a blade passing frequency (BPF) with superposition of high harmonic waves. The rotary noise will be increased due to a resonant chamber formed among the housing and every two adjacent blades of the impeller. The other one is vortex noise, which has a broad spectrum. When rotating, the impeller drives air to generate vortex, and a Karman vortex street is defined between every two adjacent blades of the impeller.
What is needed, therefore, is an impeller and a cooling fan incorporating the impeller which can overcome the above-mentioned disadvantage.